There are three chat channels on the server: global, region, and local. Global chat will be seen by any player who is in the global channel, across the entire server. Region chat can been seen by any player who owns a plot in your region, and is in the region channel.. You can only use region chat if you own a plot. Local chat can be seen by players within 50 blocks of you who are in the local channel.

/tell player message
Sends a player a private message.
/reply message
Replies to a private message.
Toggles your chat on and off.
Toggles your private messages on and off.
Ignores a player. You will no longer see that player's chat and they will not be able to private message you.
Unignores a player.

Chat Channels

/join channel
Join one of the channels and immediately begin talking in that channel.
/leave channel
Leave a channel.
/listen channel
Listen to a channel without switching to talk in that channel.
/talk channel
Change the channel you are talking in.
If you are not in global, join global and switch to talk in global. If you are in global, leave global.